I don’t know very much about the 19th century poet Lord Byron, so this will not have much of a historical perspective, and is mostly about the 2003 movie Byron. But it was rumored, during this lifetime, and continues to be a theory today, that he had an incestuous affair with his half-sister Augusta. (She was the daughter of his father’s first wife, and was five years older than him. They did not grow up together – Augusta was raised by her grandmother and other friends and relatives after her mother died. But they knew each other.) It is even rumored that one of Augusta’s children, Elizabeth Medora Leigh, was actually fathered by Byron and not by her husband. I don’t know very much about where these rumors or the evidence for these theories comes from, but apparently Byron went to visit Augusta and the newly-born Medora and wrote in a latter to a friend, “Oh, but it is not an ape, and it is worth while”, which, if he really did write that, seems pretty conclusive to me. I gather Medora is the name of a character from a poem written by Byron during a period of weeks when he and Augusta were trapped together at his home by snow.
It would seem they grew to be very close, whether it was an incestuous relationship or not. They wrote a lot of letters to each other, and Bryon’s only legitimate daughter (with his wife Annabella) was named Ada Augusta. Also, Bryon wrote “Stanzas to Augusta” and “Epistle to Augusta”. They are not considered to be major works by him, but it’s possible the rumors of incest are one of the reasons why they are not particularly popular compared to some of his other products. I won’t set about interpreting them, except to say they make it obvious that he loves her very much.
Oh, blest be thine unbroken light!
That watched me as a seraph’s eye,
And stood between me and the night,
For ever shining sweetly nigh. (from “Stanzas”)
My sister! my sweet sister! if a name
Dearer and purer were, it should be thine;
Mountains and seas divide us, but I claim
No tears, but tenderness to answer mine:
Go where I will, to me thou art the same –
A loved regret which I would not resign.
There yet are two things in my destiny, –
A world to roam through, and a home with thee. (from “Epistles”)
I don’t know for myself, but according to this anon, most movies about Byron either exclude Augusta, or exclude the incest. The exception seems to be Byron (2003), a British made-for-tv two-part miniseries starring Jonny Lee Miller as Byron and Natasha Little as Augusta. I recently watched it and can confirm that Byron/Augusta was canon as canon can be (in fact, a very fine example of it), and that the miniseries itself was good.